1. Field of the Invention
The field of the invention is directed to light emitting devices with phosphor wavelength conversion and methods of producing the same. More specifically, the invention concerns light emitting devices based on light emitting diodes (LEDs) and methods of coating one or more phosphor materials on an LED chip.
2. Description of the Related Art
White light generating LEDs (“white LEDs”) are a relatively recent innovation and offer the potential for a whole new generation of energy efficient lighting systems to come into existence. It is predicted that white LEDs could replace filament (incandescent), fluorescent and compact fluorescent light sources due to their long operating lifetimes, potentially many 100,000 of hours, and their high efficiency in terms of low power consumption. It was not until LEDs emitting in the blue/ultraviolet part of the electromagnetic spectrum were developed that it became practical to develop white light sources based on LEDs. As taught, for example in U.S. Pat. No. 5,998,925, white LEDs include one or more phosphor materials, that is photo-luminescent materials, which absorb a portion of the radiation emitted by the LED and re-emit radiation of a different color (wavelength). Typically, the LED chip (die) generates blue light and the phosphor(s) absorbs a percentage of the blue light and re-emits yellow light or a combination of green and red light, green and yellow light or yellow and red light. The portion of the blue light generated by the LED that is not absorbed by the phosphor is combined with the light emitted by the phosphor to provide light which appears to the human eye as being nearly white in color.
FIG. 1 illustrates the fabrication of such a prior art white LED 10 in which the LED chip 12 is packaged in a cup-shaped package (housing) 14. The LED chip 12 is mounted to the floor of the cup (recess) and the LED chip's electrode pads are electrically connected to corresponding electrodes 16 on the floor of the package using bond wires 18. The cup (recess) is then filled with a transparent polymer material 20, often silicone, which is loaded with the powdered phosphor material(s) such that the entire surface of the LED chip is covered by the phosphor/polymer mixture. Typically the inner walls of the cup are inclined and are reflective to enhance the emission brightness of the device. Often, a lens (not shown), whose dimensions correspond to the dimensions of the cup, is then mounted on the package to focus the output light.
A problem with existing light emitting devices with phosphor wavelength conversion is that the color/correlated color temperature (CCT) of light emitted by such devices can vary across the light emitting surface of the device. The color/CCT depends on the thickness of phosphor/polymer and the distance (i.e. path length) that light travels from the LED chip through the phosphor/polymer encapsulation before being emitted from the device. As shown in FIG. 1, light 22 which is emitted substantially on axis will have traveled a shorter path length 24 within the phosphor/polymer encapsulation than light 26 emitted off axis towards the periphery of the device in which the path length 28 is correspondingly longer. As a result the light 22 emitted substantially on axis will have a higher proportion of blue light compared to yellow (phosphor generated light) and will appear to be blue-white in color. Conversely light 26 emitted off axis towards the periphery of the device will have a correspondingly higher proportion of yellow light emitted by the phosphor and will appear yellow-white in color. For general lighting applications, where for example a diffuser is used, this variation in color is not a problem as the lit object itself will also increase illumination color uniformity.
Furthermore, the inventors have appreciated that in applications in which the LED includes further optical components, in particular a lens, to focus the output light, such color/CCT variation can become a significant problem. For example for a white LED which includes a lens, the focused light spot will have a blue-white core with a pronounced yellow-white periphery. Moreover, the inventors have further appreciated that a cause of this degradation in color/CCT uniformity results from the LED's poor approximation to an ideal point source. Typically the cup, which is often circular or square in form, is a few millimeters (e.g. 3.5 mm) in dimension as compared to the light emitting surface of the chip which is relatively smaller and may be in a range of a few tens to hundreds of micrometers (50 to 500 μm) and up to one millimeter in size. Once the cup is potted with the phosphor/polymer mixture the effective light generating area of the LED becomes the size of the cup which then corresponds to the size of the lens.
In addition to the problem of non-uniformity in emitted color/CCT due to the variation in path length through the phosphor/polymer encapsulation, the inventors have discovered that the phosphor material(s) can accumulate unevenly during curing of the liquid polymer resulting in a non-uniform distribution of the phosphor material(s) over the LED chip and in particular on the edges of the LED chip, which will also emit light, where there may be little or no phosphor material(s). As illustrated in FIG. 1 the phosphor material can accumulate on the bond wires 30, on the upper surface 32 of the LED chip, on the floor 34 of the cup (recess) and on the inclined reflecting walls 36 of the package. To overcome this problem a greater quantity of phosphor material is often used though this will result in a corresponding decrease in emitted light intensity.
US 2006/0097621 teaches a method of manufacturing a white LED comprising dispensing droplets of a high viscosity liquid phosphor paste on an upper surface of the LED such that the phosphor paste is applied onto the upper surface and side surfaces of the LED and then curing the phosphor paste. The phosphor paste comprises a phosphor powder mixed with a transparent polymer resin and has a viscosity of 500˜10,000 cps. The volume of the phosphor paste droplet and viscosity of the phosphor paste are selected such that the phosphor paste covers the upper surface and side surfaces of the LED and allows the phosphor paste to be uniformly applied to the side surfaces as well as the upper surface. After application of the phosphor paste the polymer resin is cured and the LED chip is connected to the package using bond wires. Finally the package is filled with a transparent polymer material to protect the bond wires. The inventors of the present invention consider that a limitation of such a method is that due to the high viscosity of the phosphor paste the manufacturing method will be too slow for viable mass production.
The present invention arose in an endeavor to reduce the physical dimensions of the light generating surface area of light emitting devices with phosphor wavelength conversion and to improve the color uniformity of the light emitted by such devices.